1. Field of the Invention
The subject matter of the present invention includes optical glasses with indices of refraction nd≧1.70, with Abbé numbers νd≧35 and with densities ρ≦4.5 g/cm3, optical elements or components made from these optical glasses with these properties and to devices including these optical elements. Also the invention includes compositions for making glasses having these properties.
2. Description of the Related Art
The current trend in optical data transfer/imaging/telecommunications fields is in the direction of ever-smaller device layouts and higher data transfer rates. Also the traditional “read-only” technology devices in all sectors of these fields are being forced out ever more completely by “read-write” technology devices. The specifications for optical materials and optical systems are always changing because of these considerations and trends.
Read-only-technologies can be operated in monochromatic modes in separate operations (reading or writing) rigorously separated from each other in both time and space. Because of this separation the writing process can be performed with light of the same wavelength as the reading process that occurs during a later operation. However this is not possible in “read-write” technology. In this latter technology the wavelength of the energy-rich write beam must be about 2 to 5 nm lower than that of the energy-poor read beam. Otherwise the apparatus cannot operate both modes in the same optical head and the system expenses and costs of the apparatus become unacceptably large.
This wavelength difference results from the necessity of maintaining the read beam and the write beam in the optical system cleanly separated from each other, in order to prevent serious imaging errors because of interference and residual effects. The smaller the difference between both wavelengths that is required in order to maintain a complete separation, the simpler is the resulting optical system. The term “simpler” relates to both the minimum size of the module and also the costs involved in making it.
The minimum wavelength difference required for complete separation however depends on the dispersion of the glass components of the optical system. The higher the dispersion, also the less the Abbé number, the more both never-ideally-monochromatic beams are spatially spread out, until they finally overlap which hinders the desired separation. This means that the less the dispersion of the glass, the smaller the wavelength difference that can be tolerated while maintaining complete separation. One additional advantage results from a smaller dispersion: Definitely smaller wavelengths can be used given an equal minimum wavelength difference. Generally the dispersion of the beams is greater at smaller wavelengths. Generally the minimum allowable operating wavelength increases, as well as the minimum wavelength difference, for glasses with higher dispersion in contrast to glasses with lower dispersion. A working range with lower wavelengths is preferable in many fields, since the smaller the working wavelengths are, the more information per unit surface area can be packed onto a data recording media. When the information density is maximized, the resulting shorter beam paths minimize retrieval times.
The index of refraction is of significance for practical optical systems. The individual pick-up lenses determine the focal length or distance of the system decisively because of their indices of refraction as well as the absolute wavelength of the read-write beams. The smaller the focal length of the optical system, the smaller are its spatial dimensions and thus the component size as well as the weight and cost are less. A higher index or refraction is essential in the wavelength range decisive for the particular application. An additional advantage of the high index of refraction is that the aspheric coating of the pick-up lenses can be comparatively thin. Smaller index of refraction values for the glass would require large coating thickness in order to obtain the desired effect. The required coating thickness directly affects the number of process steps required for the coating process as a parameter and thus enters into the costs and expenses. High transmission in the working wavelength range of the system is regarded as essential. The less the transmission of the glass at the working wavelengths, the poorer is the light yield of the system. Since the light intensity however enters directly into the read-write quality of the system, light sources of higher power are required. Also the cooling unit connected with the light source must be of higher power. Thus costs and work increase.
Besides the above-mentioned optical properties also additional physical and chemical properties of the glasses are important. These properties include a reduced density and a satisfactory ability to be coated, especially with organic materials.
The density of the optical materials of these systems is of great significance. The pick-up lenses acting as components of the read-write head are moving elements of the system. The head moves over the recording medium for individual data transfer operations. The retrieval time and track densities thus depend on rapid and exact positioning of the head. The greater the density of the glass components, the greater the mass of this mobile unit. Thus it has greater inertia and is more slowly put in position for the data transfer.
The aspheric coating of the lenses usually includes organic material. In order to obtain sufficient adherence of the optical coating on the base glass, the material must contain organic binding or strongly adhesion-promoting ingredients in sufficient amounts.
The patent literature already includes a few references, in which glasses with values for the optical properties nd and νd having the above-mentioned ranges or limits are described. However those glasses have a number of different disadvantages as explained in more detail hereinbelow.
DE-AS 10 61 976 describes glasses from the three-component system B2O3—SiO2—La2O3. These glasses, comprising green glass and numerous additional components, vary strongly in their compositions. They are however both GeO2 and also Yb2O3 free. They are not easily coated however. The same goes for the glasses of DE 31 02 690 A1, for the glasses of DE-AS 26 52 747 from the B2O3—La2O3—Y2O3—TiO2 system. It is also true for the glasses from the B2O3—Gd2O3—La2O3 system.
The glasses of JP 53-25323 A have in contrast a high Yb2O3 content of up to 40 percent by weight and turning to the examples a high Ta2O5 content or a high alkaline earth oxide content. Because of the extremely high Yb2O3 these glasses have a strong inclination to crystallize.
The glasses described in U.S. Pat. No. 4,166,748 and JP 59-1955523 A do not have an advantageous combination of low density, low dispersion and large index of refraction.